This project is named "Ghost Knocker". You will get the idea if you watch this movie.
Step 1: Check the materials you need.
I used an Arduino Pro Mini 3.3V. You also need USB-Serial tether to program it.
2. A battery box
I used a AA x 4 battery box with power line wires.
I used a mini breadboard by Sparkfun.
4. ECM (Electet Microphone) Module
I used an ECM module by Sparkfun.
5. RC Servo motor
I used a mini servo motor EK2-0508 by E-Sky.
6. Jumper wires
7. An Aluminum Electrolytic Capacitor
I used a 100uF capacitor.
8. A knocker hammer parts
I used a small wooden ball toy.
9. A flexible metal wire
It is for connecting the servo horn and the knocker hammer.
A steel wire 2mm in diameter will be suitable.
10. Double-stick tape
It is for making the parts bond, and for mounting the device on the door.
Step 2: Make the knocker hammer.
Make the wire to be in hole and bend it to fix on the servo horn.
Bond the ball (or whatever you choose for hammer) to the wire.
Drill the hammer if you need.
Step 3: Mount the breadboard and the servo on the battery box.
And make the RC servo fix on the battery box to move the servo horn can move.